Most color cathode ray tubes (CRTs) employ an inline electron gun arrangement for directing a plurality of electron beams on the phosphor-bearing inner screen of the CRT face-plate. Most color CRTs also employ a self-converging magnetic deflection yoke for positioning each of the electron beams in common alignment as they are swept across the CRT faceplate in a synchronous manner. The self-converging deflection yoke applies a non-uniform magnetic field to the electron beams giving rise to an undesirable astigmatism in and defocusing of the electron beam spot displayed on the CRT's faceplate. In general, the magnetic field of the self-converging deflection yoke includes a dipole component and a quadrupole component. The dipole component deflects the beam in a desired direction (either horizontally or vertically in a raster-like manner), while the quadrupole component converges the three electron beams at all locations on the CRT screen as the beams are displaced across the screen. The self-converging characteristic of the magnetic quadrupole field causes the self-converging deflection yoke to exert a negative astigmatism factor on the electron beams resulting in an under-focusing of each of the beams in the horizontal direction and an over-focusing of the beams in the vertical direction.
Referring to FIG. 1, there is shown the general shape of an electron beam spot 22 on the phosphor-bearing display screen 20 of a CRT. The self-converging magnetic deflection yoke provides a non-uniform magnetic field having a strong pin cushion-like horizontal deflection magnetic field and a strong barrel-like vertical deflection magnetic field to converge the electron beams on the peripheral portion of screen 22. As the electron beams pass through the non-uniform magnetic field, the three beams are subjected to distortion and defocusing. This distortion and defocusing increases with increasing beam deflection angles. Thus, the electron beam spot 22 shown with cross-hatching in the center of screen 20 is generally circular in cross-section, while electron beam spot becomes elongated and non-circular with increasing beam deflection as shown in the top, side and corner portions of the display screen. The beam spot thus becomes horizontally elongated when deflected along the horizontal axis and becomes both horizontally and vertically elongated in the corners of the display screen 20 such that the electron beam spot assumes a generally elliptical shape with halo-shaped elongations 24 thereabout. The halo-shaped elongations 24 are of reduced peak brightness and degrade video image resolution at large beam deflections.
As shown in FIG. 1, even where there is no halo-shaped elongations 24 extending from an electron beam spot 22, such as along the vertical and horizontal center lines of the display screen 20, the beam spot still suffers from ellipticity which limits video image resolution. With reference to FIG. 2, there is shown a comparison of the length of an elliptically-shaped beam spot, d.sub.H1, with the diameter, d.sub.H2, of a circular beam spot, where d.sub.H1 &gt;d.sub.H2. The electron beam astigmatism shown in the beam spot of FIG. 2 is defined in terms of the difference between the horizontal focus voltage and the vertical focus voltage, or: EQU Astigmatism=V.sub.FH -V.sub.FV
where
V.sub.FH =horizontal focus voltage, and PA0 V.sub.FV =vertical focus voltage.
Referring to FIGS. 3a and 3b, there is shown graphically the variation of electron beam spot size, D.sub.S, with changes in horizontal focus voltage, V.sub.FH, and vertical focus voltage, V.sub.FV. As shown in FIG. 3a, with the electron beam spot at the center of the display screen, V.sub.FH =V.sub.FV and electron beam astigmatism is zero with the beam spot having a generally circular cross-section. As the electron beam is deflected from the center of the display screen, the horizontal and vertical focus voltages change in value, with V.sub.FV assuming greater values than V.sub.FH as shown in FIG. 3b. Where V.sub.FV &gt;V.sub.FH, the electron beam experiences a negative astigmatism assuming the elliptical cross-sectional shape shown in FIG. 3b.
Prior attempts to eliminate this negative astigmatism and deflection defocusing caused by the self-converging deflection yoke have made use of a dynamic electrostatic quadrupole lens in the main lens portion of the electron gun which is oriented 90.degree. from the self-converging yoke's quadrupole field. A dynamic voltage, synchronized with electron beam deflection, is applied to the quadrupole lens to compensate for the astigmatism caused by the deflection system. The quadrupole lens exerts a dynamic positive astigmatism, which is in phase with, but has an opposite polarity from, the yoke's negative astigmatism for dynamic focusing of the electron beams over the CRT screen. The astigmatism of the electron beams caused by the quadrupole lens tends to offset the astigmatism caused by the color CRT's self-converging deflection yoke. To date, dynamic quadrupole lenses are capable of only improving deflected spot size in the vertical direction and offer no improvement in deflected beam horizontal spot size. This is because the self-converging deflection yoke over-focuses the electron beam in the vertical direction and the horizontal outer rays cause the problem. An electrostatic quadrupole can effectively converge these outer horizontal rays, but in the horizontal direction it is the inner rays which give rise to electron beam astigmatism and a dynamic electrostatic quadrupole has minimum effect on the inner rays of the beam.
This is shown in FIGS. 4a, 4b and 4c. FIG. 4a shows the location of inner and outer electron beam rays in the deflection yoke plane and at the display screen without the negative astigmatism of a self-converging magnetic deflection yoke. Without the self-converging deflection yoke effect, the outer electron beam rays meet the inner electron beam rays at the screen and the electron beam rays are in focus. FIG. 4b illustrates the situation in the horizontal plane where the self-converging deflection yoke applies a negative astigmatism to the electron beam and under-focuses the electron beam in the horizontal direction. With the electron beam horizontally under-focused, the inner rays form an image which is larger than that of the outer rays. The electron beam spot thus becomes horizontally elongated when deflected along the horizontal axis by the self-converging deflection yoke. In the vertical plane, the electron beam is over-focused by the self-converging deflection yoke as shown in FIG. 4c where the outer rays are displaced further from each other than the inner rays and thus form a larger image along a vertical direction.
Other prior approaches have exerted a fixed positive asymmetric correction factor on the electron beams in the beam forming region (BFR) of the electron gun. This approach generally exerts a fixed positive astigmatism on the electron beams to offset the negative astigmatism imposed by the self-converging yoke on the deflected electron beams. The negative astigmatism of the self-converging yoke used with the inline electron gun varies with yoke current and increases to a maximum at full beam deflection in the corners of the CRT screen and reduces to zero with the beams at the center of the screen. Thus, because the self-converging deflection yoke's astigmatism varies with time and the positive asymmetric correction applied in the BFR of the electron gun is fixed, this approach is a compromise and does not provide astigmatism correction over the entire display screen. This approach over-corrects at the center and under-corrects at the corners.
The aforementioned problems encountered in the prior art cause even more serious problems in high resolution color CRTs such as those having a flat faceplate and foil tension shadow mask, where the flat geometry imposes substantially greater challenges than those encountered with a curved faceplate. The present invention addresses the aforementioned problems of the prior art by reducing an electron beam bundle's horizontal cross-section such as by imposing a negative astigmatism in the CRT's beam forming region so that the deflected electron beam spot experiences less horizontal under-focusing effect from the self-converging deflection yoke for improved beam spot horizontal resolution.